Bi-fuel vehicles fuelled with a liquid fuel, such as gasoline or ethanol, as one fuel, and alternatively with a gaseous fuel, such as compressed natural gas (CNG) or liquefied petroleum gas (LPG), historically have had limited penetration into the consumer automobile market. More recently, growing market demand has led automobile original equipment manufacturers (OEMs) to invest more in developing bi-fuel vehicles as a product. Two major reasons influencing this trend include commodity prices and emissions standards.
Decisions for automobile purchases are directly affected by the relationship between the prices of crude oil versus natural gas. Manufacturers of automobiles are responsive to these decisions and accordingly are indirectly influenced by this relationship. That is, fuel costs influence how consumers will invest in automobiles that consume natural gas or fuels derived from crude oil, such as gasoline and diesel. Historically, the prices of crude oil and natural gas generally maintained a 10-to-1 relationship, so that one barrel of crude oil was priced at roughly 10 times one million British thermal units of natural gas. Energy parity is approximately a 6-to-1 ratio, implying that other barriers, such as infrastructure logistics, must be factored into the equation even though energy derived from crude is more expensive than natural gas. More recently, this relationship has increased by about 100% to a 20-to-1 ratio. Suddenly, consumers are more willing to consider alternative fuel vehicles, for example so called bi-fuel, dual fuel or multi-fuel type vehicles, in large part because of the much higher fuel costs associated with gasoline or diesel.
A dual fuel engine is defined herein to be an engine that can be fuelled with two different fuels at the same time, whereas a bi-fuel engine is defined herein to be an engine that can be fuelled with either one fuel or another fuel, and a flexible-fuel engine is defined herein to be an engine that operates either as a bi-fuel or a dual fuel engine. There is a need for a new and improved apparatus and method for delivering fuel to a combustion chamber of a flexible-fuel engine.
Emissions standards are regulatory requirements that set specific limits to the amount of pollutants that can be released into the environment from the operation of a motor vehicle. These standards specifically restrict emissions of carbon monoxide (CO), oxides of nitrogen (NOx), particulate matter (PM), formaldehyde (HCHO), and non-methane organic gases (NMOG) or non-methane hydrocarbons (NMHC). The limits are typically defined in grams per kilometer (g/km). Since the introduction of catalytic converters and the corresponding phase-out of leaded gasoline in most of the world, great improvements have been made towards reducing pollution derived from automobiles. Over time, and with technology advances, emissions standards become increasingly more stringent. For example, in the United States an automobile manufacturer's combined fuel economy for their entire fleet must now meet average targets, and more recently these targets include greenhouse gas emissions. In order to meet new regulatory requirements improvements were sought in engine control system technologies and in the catalytic converters that reduce harmful by-products from combustion of liquid fuels. However, as standards for emissions are continually becoming more stringent, manufacturers are finding it more difficult to meet these standards with catalytic converters alone, or with changes to well established engine control systems.
Natural gas is the cleanest of all the broadly available fossil fuels. The main products of the combustion of natural gas are carbon dioxide and water vapor. Gasoline is composed of more complex molecules, with a higher carbon ratio and higher nitrogen and sulfur contents. As gasoline is combusted there are higher levels of carbon emissions, nitrogen oxides (NOx), sulfur dioxide (SO2) and particulate matter (soot) compared to the by-products of natural gas combustion.
Improvements in emissions are obtained if a vehicle is fuelled at least some of the time with natural gas. Automobile manufacturers are now considering alternative fuel vehicles, and especially bi-fuel vehicles fuelled with natural gas as one fuel or gasoline as another fuel, as a means for meeting current and future emissions standards, as emission reductions achieved by catalytic converters are approaching the current paradigm limit and further improvements in such converters are more difficult to obtain. The present-day fuelling infrastructure for natural gas is not as well developed as that for gasoline and diesel, so bi-fuel vehicles allow operation in areas where an operator might be at risk of running out of fuel if natural gas were the only fuel the vehicle could use.
After-market bi-fuel vehicles have been in use for some time. Conventionally, standard gasoline vehicles are retrofitted in specialized shops, which involve installing compressed natural gas (CNG) cylinders in the trunk to serve as fuel tanks and the installation of an injection system and electronics on the engine. The performance and emissions of these vehicles are less than optimal due to a limited cooperation between the original engine system and the aftermarket system. Gasoline vehicles converted to run on natural gas suffer a performance penalty due to the low compression ratio of the gasoline engines, resulting in a reduction of delivered power (10%-15%) while running on natural gas. Such bi-fuel vehicles are optimized to operate with gasoline and are typically less efficient when fuelled with natural gas.
After market dual-fuel vehicles conventionally employed a fumigation conversion kit or an injection conversion kit. Prior to on board vehicle computers, for example on-board diagnostics (OBD) systems, fumigation conversion kits were used with a mixer and a regulator for non-injection systems. With the introduction of fuel injection and on board diagnostics into standard vehicles, conversion kits evolved into port injection techniques that interoperate, though in a limited fashion, with the original vehicle manufacturers' fuelling strategy and sensor system checks. Again, both conversion techniques were sub-optimal solutions due to limited cooperation between the original engine system and the conversion kit, the compression ratios employed, and due to performance limitations inherent in low pressure natural gas introduction through the intake valve.
The introduction of gasoline into cylinders for combustion has progressed due to advances in technology from being blended with air in a carburetor to being port injected into intake ports, both methods by which gasoline is introduced into the combustion chambers as part of the intake charge. The latest development has been injecting gasoline directly into the cylinders. Direct injection pressures are very high, for example 30,000 pounds per square inch (psi) (206,842.7 kilopascals (kPa)), in order to overcome in-cylinder pressure and to atomize the gasoline as it is injected to improve combustion efficiency. Gasoline being a liquid fuel is an incompressible fluid and is easily and quickly pressurized to the required pressure for direct injection. Because of the relatively high pressure differential between fuel rail pressure and in-cylinder pressure, the fuel flow rate is controllable and predictable. By controlling the amount of fuel delivered to the cylinder the amount of power created from combustion can also be controlled. Higher compression ratios are allowed in direct injection engines with less danger of knocking, defined as the premature ignition of fuel in the combustion chamber. Direct injection also means that the fuel does not displace air from the intake charge drawn into the combustion chamber through the intake ports.
Since gaseous fuels like natural gas are compressible fluids it is more difficult to manage higher injection pressures and there is an energy penalty associated with compressing gaseous fuels to higher pressures. Accordingly, conventional gaseous fuel systems have favored relatively low pressure injection systems. For example, an injection pressure in the range of 30 to 300 psi (206.8 to 2,068.4 kPa) involves fewer technical challenges than injection at high pressure and is adequate for injection into the intake air stream. After market systems typically employ low pressure port injection strategies for natural gas in dual-fuel and bi-fuel vehicles. However, because the fuel is pre-mixed with the intake air, natural gas spark ignition engines operate at modest compression ratios in the range of 9:1 to 12:1, in order to prevent engine knock, which can cause serious engine damage. Compared to engines with higher compression ratios, these engines operate at lower brake mean effective pressure (BMEP) and peak pressure levels.
High pressure direct injection of natural gas, that is, injection beginning late in the compression stroke, for example 20° before and after top dead center, involves greater technical challenges in the fuelling system. For engines operating with this architecture, the natural gas fuel rail pressure is on the order of 3,000 psi (20,684.3 kPa). This pressure is not as high as liquid fuels because there is no need to atomize a gaseous fuel, but the pressure still needs to be high enough to overcome the in-cylinder pressure and to allow fuel flow rates high enough to inject the required amount of fuel in the time available. However, even at this relatively low injection pressure, compared to liquid fuels, there is still a significant energy penalty for pressurizing the fuel and there is a significant capital cost associated with equipment needed to raise the gaseous fuel pressure. The high pressure equipment includes fuel compressors and fuel injectors. Designing high pressure, natural gas fuel injectors that inject a precise quantity of fuel into the combustion chamber has technical challenges not associated with low pressure, natural gas injection. The high pressure injection window for natural gas is typically smaller than in low pressure injection. It is known that as the on-time of the injector is decreased ballistic mode effects in the injector can decrease the accuracy of the quantity of fuel delivered. These factors are no deterrent for large heavy duty vehicles that use a lot of fuel and that require higher efficiency and higher torque. However, these same factors can deter the acceptance of this technology for light duty vehicles which consume less fuel and have lower power requirements.
Bi-fuel vehicles have traditionally been gasoline fuelled vehicles adapted to be capable of being fuelled with a different fuel. This has resulted in the most current gasoline fuel systems being combined with a gaseous fuel system. Now that the latest designs for gasoline engines use injectors to inject gasoline directly into the combustion chamber the problem to be solved has been designing a complementary fuel system for operation using another fuel, like natural gas. A typical solution would be to add the natural gas upstream of the combustion chamber, for example using port injectors. However, when combining conventional direct-injection gasoline injectors and either port or direct injectors of natural gas in bi-fuel vehicles, under normal operating conditions, the gasoline injectors are subjected to intense heat. When operating with gasoline, the gasoline fuel injectors are cooled, in part, by liquid fuel running through them. This cooling does not happen when operating in CNG mode for extended periods of time. Then, the uncooled gasoline injectors heat up and can become damaged. Additionally, liquid fuel held inside the charged injectors begins to form deposits which tend to restrict the flow of fuel, with this adversely affecting injector behavior. The longer CNG mode continues with dormant gasoline injectors the greater the risk for accumulation and hardening of deposits inside the gasoline injector.
U.S. Pat. No. 7,832,381, issued Nov. 16, 2010 to Pott et al., discloses a method of operating an internal combustion engine of a motor vehicle, that selectively uses gasoline or ethanol by direct injection into combustion chambers, and optionally instead of or in addition to injection of gasoline or ethanol the internal combustion engine is operated with a gaseous fuel, for example compressed natural gas (CNG) or liquefied petroleum gas (LPG) which is introduced with the intake air. Pott et al. teach that fouling of direct-injection gasoline injectors in CNG gas mode is monitored by this method and damage to the gasoline injectors is avoided by means of periodic changeovers to gasoline mode of operation, so that the flow of gasoline through the injectors acts to keep them cool. This method results in greater use of gasoline in order to maintain the integrity of the gasoline injectors, which increases emissions of pollutants from combustion and which can lead to greater fuel costs and more frequent trips to fuelling stations in order to maintain the fuel available in multiple fuel tanks.
European Patent Publication No. EP 2,009,277A1, published on Dec. 31, 2008 for Mats Morén, discloses an engine system with injection of a liquid fuel directly into the combustion chamber through liquid fuel injectors, and gaseous fuel injectors arranged to inject gaseous fuel into the intake port of the engine. Further, the engine system comprises means to selectively inhibit supply of the liquid fuel to the liquid fuel injectors. The gaseous fuel supply system is arranged to communicate with the liquid fuel injectors through inter-fuel system conduits, fuel pressure sensors and fuel conduit shut-off valves so that the gaseous fuel can be temporarily directed to the liquid fuel injectors in order to purge remaining liquid fuel therefrom during switchover to a gaseous fuel mode of operation.
The present apparatus and method provide improved delivery of fuel to a flexible-fuel internal combustion engine.